1. Field of the Invention
The present invention relates to an image reading apparatus that reads image information recorded on an original and control method thereof.
2. Description of the Related Art
Some conventionally known image reading apparatuses that are used in copiers and the like perform what is called “reading while feeding”. In such an apparatus, the auto document feeder conveys an original sheet by sheet onto the platen glass. The image sensor, such as a CMOS or CCD, reads original images while the exposing unit emits light onto the original on the platen glass.
However, if a piece of dust is attached to the image reading position of the platen glass, a defect image in the form of a string (streak) may occur in the read images. The term, “streak” refers to an image that occurs in the same main scanning position in a read image, and thus is different from string-shaped thin lines that originally exist in an original. Herein, such a streak resulting from a piece of dust is referred to as a “dust image”. The dust image can appear in the form of a white string or a black string, depending on the background of the original.
An invention is known that detects a dust image from images in a read page (Japanese Patent Laid-Open No. 2005-117090). Specifically, an image in which a predetermined number of sequential pixels that have no correlation with (have a large difference in brightness from) adjacent pixels in the main scanning direction exist in the sub-scanning direction is determined as a dust image.
However, according to the invention disclosed in Japanese Patent Laid-Open No. 2005-117090, if a situation occurs in which a dust image extending in the sub-scanning direction and an original image extending in the main scanning direction intersect each other, because there is a correlation (a small difference in brightness) between a pixel of interest and adjacent pixels located in the main scanning direction (the difference in brightness is small), an erroneous determination may be made that the dust image has disappeared in the sub-scanning position.
FIG. 11 shows an example of dust image detection according to the related art. In a diagram 1101, a dust image 3001 has occurred in a read original image. Note that the dust image 3001 is overlaid on a character 3002. A diagram 1102 shows the brightness of a sub-scanning position A. It can be seen from the diagram 1102 that the brightness of a main scanning position that corresponds to the dust image 3001 is decreased relative to that of the periphery. That is, there is a pixel that has no correlation with adjacent pixels in the main scanning direction. An image in which a predetermined number of such pixels that have no correlation with adjacent pixels exist sequentially in the sub-scanning direction is determined as a dust image. A diagram 1103 shows the brightness of a sub-scanning position B. It can be seen from the diagram 1103 that the brightness of a main scanning position that corresponds to the dust image 3001 decreases slightly relative to that of the periphery. However, because the character 3002 exists, the brightness of the peripheral main scanning positions also decreases. That is, because there is a correlation between a pixel of interest and adjacent pixels located in the main scanning direction, an erroneous determination may be made that the dust image has disappeared in the sub-scanning position B. A diagram 1104 shows the brightness of a sub-scanning position C. It can be seen from the diagram 1104 that the brightness of a main scanning position that corresponds to the dust image 3001 decreases relative to that of the periphery, indicating that a pixel that has no correlation with adjacent pixels located in the main scanning direction has appeared again.
A diagram 1105 shows an example of a method for correcting a dust image according to the related art. According to the related art, a dust image 3001 is detected based on the continuity of the dust image, and a dust image that extends from the detected dust image is corrected with peripheral pixels. Accordingly, in the case where a dust image is overlaid on a character 3002 or a horizontal ruled line in an original, an erroneous determination may be made that the dust image has disappeared, and dust image correction may not be performed until another continuous dust image is detected. In this case, as shown in the diagram 1105, short dust images 3003 that were not corrected remain in the resulting image at portions where the dust image intersects with the character or horizontal ruled line. Therefore, this method is not preferable.